Electrical filter network



Feb. 2, 1943- H. M. LEWIS 2,309,852

ELECTR ICAL FILTER NETWORK Filed Ap ril 24, 1940 TELEVISION RECEIVER "v14 INVENTOR AROLD M. LEWIS ATTORNEY Patented Feb. 2, 1943 UNITED STATES PATENT OFFICE ELECTRICAL FILTER NETWORK Harold M. Lewis, Great Neck, N. Y., assignor to Hazeltine Corporation, a corporation of Delaware Application April 24, 1940, Serial No. 331,406

6 Claims.

ly been used, especially in power-supply systems,

has employed a vacuum tube having limited elec tron emission, the tube operating at current saturation for the purpose of maintaining at a substantially constant value the current through the tube, thus to render the system unresponsive to pulsations of voltage occurring atany point in the system preceding the tube. A disadvantage in the use of such a tube is that there results too great a loss of unidirectional potential across the tube by reason of its relatively high inherent resistance when operating at current saturation. The pentode type of vacuum tube has been operated with fixed grid bias also as a constant current device, but its use has the some disadvantages as have the tubes with limited electron emission. The prior art filter networks have additionally employed gas-filled diodes as parallel-connected constant-voltage regulators wherein the diode is connected directly across the output of thefilter network. Gas-filled diodes, however, have at least two disadvantages in that their operation is trequently critical and their controlling action is effective only within a narrow voltage range peculiar to the particular tube design.

It is an object of'the invention, therefore, to provide a new and improved filter network especially suitable for a direct current power-supply system and one which overcomes one or more of the above-mentioned disadvantages of such arrangements of the prior art.

It is a further object of the invention to provide a filter network for a direct current powersupply system wherein the output potential is regulated and is maintained substantially constant over a relatively wide range of supply voltage fluctuations and without substantial loss of supply voltage.

In accordance with one embodiment of the invention, an electrical filter comprises input and output terminals, the input terminals being adapted to have supplied thereto direct current having undesirable pulsations therein. A vacuum tube having a control electrode and a cathode is included in a direct current path between the input and output terminals. A transformer having primary and secondary windings is provided, being connected with the primary winding effectively in series with the pulsating current through the tube for developing a pulsating control potential in each of the windings. Means .are also provided to connect the primary and secondary windings in series with additive polarity between the cathode and control electrode of the tube to control the impedance thereof in a manner to oppose undesirable current pulsations between the output terminals.

In accordance with another embodiment of the invention, an electrical filter comprises input and output terminals, the input terminals being adapted to have supplied thereto direct current having undesirable pulsations. A vacuum tube, having two control electrodes, is included 'in the direct current path between the input terminals. The filter also includes impedance means effectively in series with the pulsating current through the tube and responsive primarily to the rate of change thereof for deriving a pulsating control potential and means for applying the control potential to both of the electrodes continuously to control the impedance of the tube to oppose undesirable current pulsetions between the output terminals.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims. of a filter network embodyin the invention as Fig. 1 is a circuit diagram, partly schematic,

of a filter network embodying the invention as used for the "purpose of better explaining the operation of the invention; and Fig. 3 represents a modification of the invention wherein a screen-grid type of tube is employed to effect increased filtering action.

Referring to Fig. 1 of the drawing, there is shown a circuit diagram of a complete direct cur rent power-supply system embodying the invention. ,This supply system is connected'to supply energy from a source of alternating current It to a signal-translating apparatus which may be a television receiver II shown schematically since it may be entirely conventional in construction and operation. The television receiver H is connected to receive television signals from an antenna system l2 and operates to convert the received signals to a visual image on the screen of a picture tube l3. Its operation requires that direct current of constant potential be supplied to its power-supply terminals I4. The conversion of the alternating current oi the source It to direct current at the terminals 14 is the function of the power-supply system embodying the invention.

The power-supplysystem includes a trans former 15 having a primary winding l3 connected to the alternating current source l through a switch l1, a high-voltage secondary winding l8, and a plurality of low-potential heater windings l9 and 20. secondary winding [8 is center-tapped at 2| to provide with a rectifier 22 a full-wave rectifier system, the output of which is connected to the input terminals 23, 24 of a filter network 25. The output of the filter network 25 is connected through output terminals 26, 21 to a load circuit which may include a resistor 28 and the termina s H of the television receiver H.

Neglecting for the moment the operation of that part of the power-supply system embodying the present invention, the circuit described constitutes, in general, a conventional power-supply system, the operation of which is well understood in the art. In brief, alternating current from the source I0 is stepped up in potential by the secondary winding I8 of the transformer l and the high voltage thus provided is rectifled by the rectifier 22. This rectified energy is in the form of pulsating direct current. However, since it is necessary that the direct current output of the power supply be of substantially constant potential and free from any form of pulsation or ripple, the pulsations occurring in the rectified energy are removed by the filter network 25 and the direct current is thereafter supplied free from undulations to the direct current input terminals H of the television receiver ll.

Referring now more particularly to that portion of the power-supply system which embodies the invention, the network 25 is a w-type impedance network having a shunt-capacitance arm connected across its input terminals 23, 24 comprising acondenser 3| of relatively large capacitance and having a second shunt-capacitance arm similarly connected across its output terminals 26, 21 comprising a condenser 32 of relatively large capacitance generally of equal or larger value than that of the condenser 3|. The series-impedance arm interposed between the shunt arms 3| and 32 and providing a direct current path from the input to the output terminals comprises a. controllable impedance device, such as the anode-cathode path of a vacuum tube 33, which may be of the triode type, and the primary winding 35 of a transformer 34. The transformer 34 has a secondary winding 36 which is connected in series with the primary Winding 35 between the cathode and control electrode of tube 33. The inherent resistance 0! the transformer primary winding is represented by the lumped resistance 30. This transformer comprises an impedance means eflectively in series with the pulsating current through the tube and is responsive to the rate of change thereof for deriving a control potential in each of its windings. The series connection between its winding, as will be shown more fully hereinafter, provides means for applying the derived control potential to the tube continuously to control its impedance in a manner to oppose undesirable current pulsations between the output terminals.

In considering the operation of the circuit just described, it will be seen that the tube 33 constitutes a variable impedance device the instantaneous value of impedance of which is con- The high-voltage.

trolled by the instantaneous value of potential supplied to its control electrode by the transformer 34. The inherent resistance 30 of the primary winding 35 of the transformer 34 is preferably made sufliciently small that it provides from the direct current flowing through the winding a suitable operating bias for the tube 33. A large proportion 'of the undulations of the rectified direct current supplied to the input terminals 23, 24 of the filter is smoothed or filtered out by the input condenser 3|. The remaining pulsation or ripple current produces undulating voltages 'in the windings of transformer 34 which are applied in series and with additive polarity to the control electrode of the tube 33 in a degenerative sense. That is, the changes in the instantaneous impedance of the tube 33 are such as to oppose instantaneous changes in the value of the current flowing through the primary winding 35 with the result that the tube 33 tends to mfintain constant the current flowing through it. This greatly reduces the magnitude of the current pulsations between the output terminals 26, 21. The condenser 32 is effective further to reduce between the output terminals 26, 21 the magnitude of plified circuit arrangement of Fig. 2 shows only the tube 33, the transformer 34 and the voltage drops occurring across various portions of the filter network. The current through the anodecathode path of tube 33 is given by the following equation:

vi+a a w ilzm r il (1) P l where,

R =anode-cathode resistance of tube 33, =amplification factor of the tube 33, and the several currents and voltages are those indicated on the figure, But,

ek=i Z1 and e =i Zm (2 where,

zizimpedance of the primary winding 35 and Zm=mutual impedance between the windings of the transformer 34.

Substituting these values of ex and e. in the first equation, v v r I": m F v l from which Z:e 1/i =R +Zl-+-pZ1+aZm (4) However,

Zz=mi+R3o (5 where,

ri=the inductive reactance of the primary winding 35, and

Rao=the inherent resistance 30 of his winding.

Substituting this value of Z1 in Equation 4,

The impedance of the serially-connected tube 33 and transformer winding 35 is thus seen from Equation 7 to include only the anode-cathode resistance R1) of tube 3.3 and the primary winding resistance R30 for direct current since the value of the impedances :1 and Zm is obviously zero for direct current. Very slow changes of current increase this impedance by the factor uRao and the inherent resistance of the transformer primary winding 35 thus improves the direct current regulation of the system. However, for current pulsations through the tube 33 and transformer 34, the value of the impedance Z as found by Equation 4- is increased not only by the mag- 'tube 33 simulates an inductance of very large value. It is at once apparent that the impedance which the tube 33 and transformer 34 present to fluctuating current is materially increased by the amplifying action of tube 33 and is considerably greater than that which these circuit elements present to a steady direct current.

Generally, there is sufficient resistance in the primary winding 35 of the transformer 34 to derive suitable operating bias for the control electrode of tube 33 in response to the magnitude of the current flowing therethrough. If the resistance of the primary winding is insufficient for this purpose, a suitable resistor may be arranged in series with the transformer primary winding such as a resistor 42 of Fig. 3, which shows a modification of the Fig. 1 circuit arrangement. Circuit elements of Fig. 3 corresponding to like elements of Fig. 1 are designated by like refer ence characters. This embodiment of the invention is similar to that of Fig.'1 except that a. screen-grid tube 33 is employed to improve still further the filtering action of the filter 25. The screen of tube 33 is connected to the junction point of a resistor 45 and a condenser 46 which are connected in series across the input terminals 23, 24, The resistor 45 not only reduces the potential applied to the screen grid of tube 33 to a suitable value, but additionally permits the condenser 46, which has a relatively low reactance compared to the impedance of the resistor 45, to apply to the screen grid the undulating voltage appearing across the transformer 34.

The operation of the embodiment of the invention of Fig. 3 is the same as that of Fig. 1 insofar as the controlling action of the transformer 34 on the impedance of tube 33' is concerned. However, the connection of the screen of tube 33' to the junction of resistor 45 and condenser 45 in the manner shown improves the filtering action since the screen so controls the impedance of tube 33 that the latter is additionally degenerative to potential fluctuations appearing across the transformer 34. Thus the normal tendency of the screen-grid type of tube of this modification to operate as a constant current device is greatly augmented by the effect of the transformer 34 in degeneratively controlling by both the control electrode and the screen the instantaneous impedance of the tube 33'. This obviously leads to materially improved over-all operation of the filter network.

above The impedance Z presented to undulating currents by the discharge device 33 and the transformer 34 of the Fig. 3 modification may be determined in the same manner as Equation 4 zamplification factor of the tube 33' under control of its screen grid.

It may benoted that the degenerative control provided by the Fig. 3 arrangement may be even further increased by breaking the connection of the condenser 46 to the resistor 42 and connecta ing this condenser directly to the control electrode of tube 33'. If this is done, the undulating voltages in the windings'of the transformer 34 are applied in series and with additive polarity through the condenser 46 to the. screen grid of the tube 33' and maximum degeneration is obtained by the action of both the control elec trode and the screen grid with the result that tube 33 presents an even greater impedance to undulations of the direct current flowing through the filter 25. The impedance 2" presented to undulating currents in this case is given by the equation:

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. An electrical filter comprising, input and output terminals, said input terminals being adapted to have supplied thereto direct current having undesirable pulsations therein, a vacuum tube having an anode, a control electrode and a cathode, said anode and said cathode being included in a direct current path between said input and said output terminals, and a transformer having primary and secondary windings, said primary winding being included in said direct current path adjacent said cathode for developing in each of said windings from said undesirable current pulsations a pulsating control potential, said primary and secondary windings being connected in series with additive polarity between said cathode and said control electrode continuously to control the instantaneous impedance of said vacuum. tube to oppose undesirable current pulsations between said output terminals.

2. Anelectrical filter comprising, input and output terminals, said input terminals being adapted to have supplied thereto direct current having undesirable pulsations therein, a vacuum tube included in a direct current path between said input and said output terminals, said vacuum tube having two control electrodes, impedance means effectively in series with the pulsating current through said tube and responsive primarily to the rate of change thereof for deriving a pulsating control potential, means for applying an operating bias to each of said control electrodes, and means for applying said control potential to both of said control electrodes continuously to control the impedance of said tube to oppose undesirable current pulsations between said output terminals.

3. An electrical filter comprising, input and output terminals, said input terminals being adapted to have supplied thereto direct current having undesirable pulsations therein, a vacuum tube included in series between said input and said output terminals, said vacuum tube having two control electrodes, impedance means responsive primarily to the rate of change or the current through said-tube for deriving a control potential, and means including a condenser for applying said control potential with degenerative polarity to both of said control electrodes continuously to control the instantaneous impedance of said tube in a manner to oppose undesirable current pulsations between said output terminals.

4. An electrical filter comprising, input and output terminals, said input terminals being adapted to have supplied thereto direct current having undesirable pulsations therein, a vacuum tube having a control electrode and a 'cathode and included in a direct current path between said input and said output terminals, a transformer having primary and secondary windings, said primary winding being connected effectively in series with the pulsating current through said tube for developing a pulsating control potential in each of said windings, and means connecting said primary and said secondary windings in series with additive polarity between said cathode and said control electrode to control the impedance of said tube in a manner to oppose undesirable current pulsations between said output terminals.

5. An electrical filter comprising, input and output terminals, said' input terminal being adapted to have supplied thereto direct current having undesirable pulsations therein, a vacuum tube included in a direct current path between said input terminals, said vacuum tube having twocontrol electrodes, impedance means efiectively in series with the pulsating current through said tube and responsive primarily to the rate of change thereof for deriving a pulsating control potential, and means for applying said control potential to both of said electrodes continuously to control the impedance of said tube to oppose undesirable current pulsations between said output terminals.

6. An electrical filter comprising, input and output terminals, said input terminals being adapted to have supplied thereto direct current having undesirable pulsations therein, a vacuum tube having a control electrode and a cathode, said vacuum tube being included in a direct current path between said input and said output terminals, a resistor, a transformer having primary and secondary windings, said primary winding and said resistor being included in series with said tube in said direct current path and responsive respectively to the rate of change and the magnitude of the current flowing therethrough for deriving a pulsating control potential in each of said windings and a bias potential across said resistor, said secondary winding being connected in series with said primary winding with additive polarity and in series with said resistor between said cathode and said control electrode for applying said bias potential and said control potential to said control electrode continuously to control the impedance of said tube to oppose undesirable current pulsations between said output terminals.

HAROLD M. LEWIS. 

